Mounting components such as semiconductor chips are mounted on wiring substrates having various shapes and structures. Semiconductor chips have become further integrated and sophisticated. This has increased the number of wiring layers and reduced the size of wires in wiring substrates on which semiconductor chips are mounted. To manufacture such a wiring substrate, for example, a single-Damascene process may be performed to form wires. The single-Damascene process etches an interlayer insulation layer to form wiring trenches and deposits copper in the wiring trenches to form a copper layer. Then, chemical mechanical polishing (CMP) is performed to remove the copper layer located above and outside the wiring trenches. This flattens the copper layer. Japanese Laid-Open Patent Publication No. 2000-332111 describes a prior art example of the single-Damascene process.